Vinyl ester resins are liquid thermosetting resins which are the reaction product of about equivalent amounts of a polyepoxide and an unsaturated monocarboxylic acid. Those resins are often used in combination with fibrous reinforcement and inert fillers to manufacture composite structures. One way to make such composite structures is to premix the resin, filler, fibrous reinforcement and other necessary additives to form a molding compound. The molding compound can then be formed into the desired shape and cured in a heated matched metal die. An improvement in the process is chemical thickening of the relatively low viscosity liquid resin to form a high viscosity gel after the resin has been mixed with all the other ingredients in the molding compound. This thickening or B-staging has several advantages. Unthickened molding compounds are sticky masses which are difficult to handle. After B-staging, they are firm solids whose surfaces are dry. In this form they are easily handled. During the molding operation, the molding compound flows within the die set to fill the die cavity. The increased viscosity of B-staged molding compounds inhibits segregation of the various components of the molding compound during flow and promotes compositional uniformity of the composite over the entire volume of the structure.
Vinyl ester resins which are now used in such B-staged compositions are especially modified during their manufacture to make B-staging possible. After the resin is formed, it is treated in an additional manufacturing step with a cyclic anhydride, such as maleic or phthalic anhydride, to form half acids by combination with the free hydroxyl groups on the vinyl ester. The resulting resin is a polyfunctional carboxylic acid which is capable of forming a reversible cross-linked network when treated with an alkaline earth oxide such as magnesium oxide. This network completely imbibes the styrene monomer contained in the resin solution forming a homogeneous gel.